1. Field of Invention
The present invention relates to an intake system of an internal combustion engine.
2. Description of the Related Art
U.S. Pat. No. 4,485,775 discloses an engine having a helically-shaped intake port, which can create a strong swirl motion in the combustion chamber when the engine is operating at a low speed and to obtain a high volumetric efficiency when the engine is operating at a high speed. This helically-shaped intake port includes a helical portion, an inlet passage portion tangentially connected to the helical portion, and a bypass passage connecting the inlet passage portion to the helix terminating portion of the helical portion. A swirl control valve actuated by an actuator is arranged in the bypass passage. The actuator includes a vacuum chamber and an atmospheric pressure chamber, the chambers being separated by a disphragm. This disphragm is connected to the swirl control valve. The vacuum chamber of the actuator is connected to the intake manifold via a check valve, which permits only the outflow of air from the vacuum chamber to the intake manifold.
When the engine is operating under a heavy load at a high speed, the vacuum chamber of the actuator is caused to open to the outside air so that the swirl control valve opens the bypass passage to the maximum extent. Contrary to this, when the engine is operating at a low speed, the vacuum chamber of the actuator is disconnected from the outside air and connected to only the intake manifold via the check valve.
When the engine is operating at a low speed under a light load, a great vacuum is produced in the intake manifold. At this time, since the check valve opens, a great vacuum is also produced in the vacuum chamber and, as a result, the swirl control valve is caused to close the bypass passage. The entire air flows into the helical portion from the inlet passage portion of the intake port and, thus, a strong swirl motion is created in the combustion chamber.
Since the check valve opens only when the level of vacuum in the intake manifold becomes greater than that of vacuum in the vacuum chamber, the vacuum chamber of the actuator is maintained at the maximum vacuum produced in the intake manifold as long as the vacuum chamber is not caused to open to the outside air. Consequently, even if the engine operating state is changed to one where the level of vacuum in the intake manifold is low after the engine is operated at a low speed under a heavy load, since the level of vacuum in the vacuum chamber of the actuator is maintained at the maximum vacuum produced in the intake manifold, the swirl control valve theoretically remains closed. That is, when the engine operating state is changed to the heavy load-low speed operating state from the light load-low speed operating state, the swirl control valve theoretically remains closed.
However, actually, since air leaks into the vacuum chamber of the actuator via, for example, the check valve, the level of vacuum in the vacuum chamber of the actuator gradually decreases. Consequently, if the engine operates at a low speed under a heavy load for a long time, it is impossible to maintain the swirl control valve at the closed position and, thus, the swirl control valve opens. In an engine using an air-fuel mixture having an approximately stoichiometric air-fuel ratio, if the swirl control valve opens when the engine is operating under a heavy load at a low speed, no particular problem occurs. However, in an engine using an extremely lean air-fuel mixture, if the swirl control valve opens when the engine is operating under a heavy load at a low speed, since swirl motion of the air-fuel mixture is weakened, the combustion deteriorates. As a result, a problem occurs in that good drivability cannot be obtained.